The invention relates to the field of automotive electrical systems. Specifically, the invention is directed to a method of limiting the rate of change of the output current supplied by a starter/alternator in the generator mode in the circumstance where the system load, i.e., demand for electrical energy, has rapidly increased. The method of the present invention limits the rate at which the output current can increase thereby limiting the possibility that the rapidly increasing electrical load could either stall or cause another operational fault in the associated I.C. engine.
The trend in automotive electrical systems has always been towards more power and higher voltages. At this time, an element of the trend involves the combination of the alternator and starter into a single IC engine driven unit. This combined starter/alternator can be driven either directly on the crankshaft of the IC engine as a part of the flywheel, on one end, or the balancer, on the other. Alternatively, the starter/alternator can be mounted for gear, belt, or chain drive from the crankshaft along with other IC engine driven components (i.e., waterpump/A/C compressor/power steering pump, etc.)
The starter/alternator has become more powerful not only for increasing power (current and voltage) but also for more rapid and more frequent starting cycles of the IC engine as enhanced operating efficiencies are sought. In pursuit of these goals, the starter/alternator has become more sophisticated in its control systems and its responsiveness to system requirements for both starter functions and generating functions.
In older fixed output alternator/generators (i.e., approximately 1000 watts in a range of pre-selected engines speeds), the excess load would simply divert to the battery, or other electrical power storage device, for increased current requirements and, similarly, excess output would also divert. [Either of these eventualities led to either dimming lights and decreasing performance of electrical components, in the case of too small an alternator and a weakened battery condition, or boiled away battery fluids in the case of too large a charging capacity!]
However, in the event a modern starter/alternator is not controlled properly, the combined starter/alternator is generally sufficiently load responsive and powerful to stall an IC engine that is operating at low or idle speeds. That is, the braking effect of the rapidly increasing requirement for electrical output from the starter/alternator may overwhelm the available torque of the I.C. engine.
The present invention is directed to controlling the rate of change in the output current of a combined starter/alternator in response to rapidly increasing demand loads such that the starter/alternator does not overload the I.C. engine resulting in a stall condition. In this method, the current in the starter/alternator motor phases and/or the output current from the starter/alternator inverter is monitored. Once the monitored current reaches a rate of change (i.e., response rate) that exceeds a predetermined threshold response rate, the rate of change of the output voltage of the starter/alternator is regulated and reduced to maintain the output at the predetermined threshold rate of output current change (response rate).